Electrically heated blanket



1946. w. BRADFORD 2,400,735

ELECTRICALLY HEATED BLANKET Filed May 16. 1944 ATTORNEYS Patented May 21, 1946 UNITED STATES PATENT OFFICE ELECTRICALLY HEATED BLANKET William Bradford, Taunton, Mass. Application May 16, 1944, Serial No. 535,843

3 Claims.

This invention relates to electrically heated blankets and has for it object to improve the control of blankets of this type in accordance with variations of the ambient temperature in the room.

The control of a blanket is essentially distinct from that of the usual heating pad. A heating pad is designed to operate generally at one or more fixed, heats, the control being in a manual 1y operated switch which preselects the temperature to be maintained by one or more themnc stats located in the pad itself. Such pads are used for local application of heat and are always intended for producing a sensation of warmth.

en electrically heated blanket, on the contrary.

should be regulated automatically to make up heat loss iron: the patient by radiation fi-zci" pumose should increase its heat output as room emperature drops. In accordance wzth the present invention, c. simple external control is provided which the room temperature c es aces to control in infinite gradations the tempen store at which the blanket itself operates. invention will now be described in connection with the accompanying drawing, in which.

Fig. i is e, diagrammatic view oi the blanket in voiving the invention;

Figs. 2 and are circuit diagrams illustrating the principles on which the blanket control ozccrates; and

.Figs. 4 and 5 are respectively side and end views of a thermostat arrangement by which the ex ternal control. may be maintained.

The blanket comprises a body i in which there is lmbeddedcn electrically heating element 2, one end of this heating element being connected by a wire 3 to an external control box indicated at The other end of the heating element is con-- nected by a wire 5 to a branching point B from which one wire 1 passes to the control box through a thermostat 8 imbedded in the blanket and another wire 9 passes to the control bot: through a thermostat heating resistance In.

The external control box contains a suitable temperature responsive device such as a bimetalpasses through the resistance l2 or makes contact in such a manner that a large amount of this resistance is included in the wire, the current passing through the heater it will be zero or a small quantity. in the first case. the blanket he element indicateddiagrammatically at H in Figs. 2 and 3. 'It also contains a resistance 12 in the line 9 which includes the thermostat heater to. A contact point on the bimetallic element makes contact with the resistance H at a place determined by the bending of the element. The

V blanket control thermostat 8 i subject to the heating effects both of the main heating coil 2 and thethermostat heater I0. It the bimetallic element bends so as to make contact with the rethermostat ll will be warmed rapidl by the auxiliary heater so that the thermostat be opened before the blanket itself has warmed up to the temperature for which the 'ti'iermcstat d set. produce a low heat condition of bicnket and ZE'SETZ lfi! thermostat will be adjusted so that occurs at high room tempesatme. its the room temperature decreases, the bending of the bimetallic element will gradually include more and more of the resistance 52 in the auxiliam heater circuit. As the current through the auxiliary heater is thus decreased, the blanket thermostat t will be controlled more i and more the temperature of the blanket it-= self rather: than the local influence of the heat-er it. In the extreme case shown in Fig, E, cor respondmg to very low room temperature, the heating coil to is out of circuit entirely and the blot operates at the maximum temperature of which it capable.

win altemstive arrangement of an external. thermostat is shcwnin Figs. 4 and 5. In this case a bimetallic strip 93 is wound in a helical form and is anchored at one end to an adjust inesleeve ll turning in a bracket 35.. The other .end of the helix is fixed to a disk IE upon which is mounted an evacuated insulating tube 11 preferably of curved form. The insulating tube com tains a small amount of mercury 18 or other con ducting liquid and has an electrode it at one end.

A second electrode 20 is secured in the other end of the tube and is connected to a resistance wire it which extends along the outer curved side or the tube. It the thermostat helix turns the tube 0 that it lies substantially horizontal as viewed in Fig. 5, the mercury will make contact directly between the electrode 18 and the electrode 20. As the helix rotates the tube to the position oi Fig. 5, the mercury will contact further down the resistance wire, causing a greater resistance to be included in the circuit andthe current through the auxiliary heater coil I to be corresponding- 1y reduced. As the helix turns the tube I! still further, contact between the'electrode l 9 and the resistance wire 2| may if desired be entirely broken or this eflect may be prevented by suitable stops 22, 23. The manner in which the form of thermostat just described controls the pad temperature is exactly the same as that described in connection with Figs. 2 and 3.

I claim:

1. An electrically heated blanket comprising a main heating coil embedded in the blanket, a thermostat in series with the blanket and embedded in the blanket in position to be heated by said coil, an auxiliary thermostat heater connected in shunt with the main coil and embedded in the blanket in position to supply heat to the thermostat, a resistance external to the blanket and connected in series with the auxiliary heater, and heat responsive means external to the blanhot and operating to vary said resistance in-- versely with the ambient temperature.

2. An electrically heated blanket comprising a main heating coil embedded in the blanket, a thermostat in series with the blanket and embedded in the blanket in position to be heated by said coil, an auxiliary thermostat heater connected in shulit with, the main coil and embedded in the blanket in position to supply heat to the thermostat, a resistance external to the blanket and connected in series with the auxiliary heater, :1. contact movable along the resistance, and heat responsive means external to the blanket and moving the contact to increase the resistance Ll.- cluded in series with said thermostat as the aim bient temperature drops. I

3. An electrically heated blanket comprising a main heating coil embedded in the blanket, a thermostat in series with the blanket and einbedded in the blanket in position to be heated by said coil, an auxiliary thermostat heater con nected in shunt with the main coil and embed in the blanket in position to supply heat to the thermostat, a bimetallic helix external to *l blanket, an insulating tube, an electrode tube, a resistance element located a length of the tube, an elect: tally cousin, uid within the tube, the cl mode, li n' 'l sistance element being nested in the auxiliary heater, the helix and the tube the length of resistame circuit as the ambient t 

